Photon-counting based spectral computer tomography (CT) makes use of direct converting material, e.g. CZT. The performance of CZT pixels can vary significantly across an array. Charge yield and transient response may differ to a great extent. These variations manifest themselves as a pixel gain inhomogeneity. Variations in transient response cause also gain differences due to ballistic deficit.
Today, energy calibration procedures are in place to resolve the relation between the application-specific integrated circuit (ASIC) threshold level and the energy of the impinging photons. The outcome of the energy calibration is an adjustment of the ASIC threshold(s) such that all pixels have the same effective energy threshold.
U.S. Pat. No. 8,350,221B2 discloses an apparatus for generating countable pulses from impinging x-ray in an imaging device, in particular in a computer tomograph, the apparatus comprising a pre-amplifying element adapted to convert a charge pulse generated by an impinging photon into an electrical signal and a shaping element having a feedback loop and adapted to convert the electrical signal into an electrical pulse, wherein a delay circuit is connected to the feedback loop such that a time during which the feedback loop collects charges of the electrical signal is extended in order to improve an amplitude of the electrical pulse at an output of the shaping element. Also disclosed are a corresponding imaging device and a corresponding method.
US20080018505A1 discloses an adaptive data acquisition circuit including an amplifier for amplifying electrical pulses generated by a detector responsive to energy incident at the detector. The adaptive data acquisition circuit also includes a counting circuit for counting amplified electrical pulses generated by the amplifier. In addition, the adaptive data acquisition circuit includes a digital logic circuit for determining a pulse parameter indicative of a pulse rate and an amount of energy present in the amplified electrical pulses and for generating a control signal responsive to the pulse parameter for controlling an operating parameter of the data acquisition circuit.
U.S. Pat. No. 8,309,933B1 discloses systems, devices, processes, and algorithms for adaptively filtering a signal output from a radiation detector and adaptively sampling the signal. A count rate of events detected by the radiation detector is estimated by a processing unit. An RC time constant of the filter and a sampling rate of an analog-to-digital converter are adjusted based on the estimated count rate. Events are continuously detected by the radiation detector and counted by the processing unit while the adjustable parameters (the RC time constant and the sampling rate) of the filter and the analog-to-digital converter are adjusted on the fly (in real time) to optimize an energy resolution of the detected events, while reducing degradation due to pile-up effects and improving efficiency of the analog-to-digital converter. The filter can be implemented through analog filters, digital filters, or a combination thereof.
US20060056576A1 discloses a method and system of counting and tagging radiation energy received by a radiation detector. The method and system are designed to dynamically control the sampling window or shaping time characteristics of a photon counting detector to accommodate variations of flux experienced by the detector so as to preserve optimum detector performance and prevent saturation during high flux conditions.
Conventional gain calibration however neglects pixel variations caused by differences in transient response. Variations due to transient response not only cause gain differences but may also affect the spectral linearity.